Surface acoustic wave (hereinbelow abbreviated to SAW) convolver elements, to which attention is paid recently as a key device in the SS communication system or a device for real time signal processing such as Fourier transformation, etc., are classified roughly into elastic type, air gap type and monolithic type, according to differences in the structure or the working principle thereof. All of them have a basic structure consisting of a pair of interdigital electrodes 1 converting electric signals into SAW and a region 3, where two SAWs propagating under an output electrode 2 in directions opposite to each other are multiplied by each other and integrated.
A product (BT product) of a time (signal processing time: T), in which the SAWs pass through the region described above, by a working frequency bandwidth B of an element corresponds to a process gain and in general it is thought that the greater the BT product is, the larger the signal processing capacity is. However, since T is determined by the size of the element, it is limited by fabrication conditions, etc. for a piezoelectric monocrystal substrate or a piezoelectric thin film, which is a starting material thereof and at present T obtained by a single SAW convolver element is about 20 .mu.sec. In order to increase the BT product, it is possible also to adopt a method, by which B is widened instead of lengthening T. In fact, it can be thought that this method is more useful, in the case where a high speed data transmission is effected in the SS communication, etc. However, e.g. in the case where an SAW convolver element is used in a real-time Fourier transformer, etc., the necessity of elements having a long T is great.
Further, even if there are no limitations in the size of the substrate, since the propagation loss of the SAW within the element increases in general with the increasing signal processing time, there is a tendency that an output signal level for a same input signal level is lowered with increasing T. In order to compensate this level lowering, an amplifying circuit is disposed in the input or the output. It is more advantageous to raise the input signal level for utilizing the S/N ratio improving effect by the process gain (BT product) of the element. However, usually, since the upper limit of the linear region in input and output characteristics of the element depends on the intensity of the SAW in the neighborhood of the input interdigital electrode, the optimum input signal level is not influenced remarkably by the length of the signal processing time and remains at an approximately same level. Compared in the optimum working state, it is impossible to avoid that the output signal level is lowered in an element having a long signal processing time.